Process for the production of aromatic di-and polycarboxylic acids



United States Patent This invention relates to a method for producing aromatic diand polycarboxylic acids by heating alkali metal salts of aromatic monocarboxylic acids.

As we have discovered earlier, the alkali metal salts of monocarboxylic acids, the carboxyl groups of which are attached to aromatic ring systems or to heterocyclic rings having an aromatic structure, can be transformed into salts of other carboxylic acids having at least two carboxyl groups in the molecule by heating the alkali metal salts of the monocarboxylic acids to elevated temperatures, which salts may then be transformed into the free acids or their derivatives in accordance with known methods. In copending application Serial No. 643,952, filed March 5, 1957, now abandoned, it is disclosed that this process produces improved results if the heating is carried out under carbon dioxide pressures above 400 atmospheres, preferably in the presence of acid-binding agents. On the basis of the yields of diand polycarboxylic acids, such as terephthalic acid, obtained in accordance with said application Serial No. 643,952, it must be assumed that new carboxyl groups are formed on the ring systems. In addition, a migration of the carboxyl groups may also take place, so that the reaction products, for example, those obtained from heterocyclic monocarboxylic acids, do not always correspond to the starting materials from the point of view of structure. As stated above, our prior process required the presence of a carbon dioxide atmosphere under a pressure of at least 400 atmospheres. Thus the process required the use of special pressure equipment.

It is an object of this invention to produce aromatic di and polycarboxylic acids from aromatic carboxylic acids in greater than theoretical yield Without the use of excessively high pressures.

Another object is to provide a method for the introduction of additional carboxyl groups into the molecule of an aromatic monocarboxylic acid without the use of hi 'h pressures.

A further object is to provide a process for the production of aromatic diand polycarboxylic acids in greater than theoretical yield without the use of a carbon dioxide atmosphere.

Another object of this invention is to provide a process for the introduction of additional carboxyl groups into the nucleus of aromatic monocarboxylic acids without the use of a carbon dioxide atmosphere.

These and other objects of our invention will become apparent as the description thereof proceeds.

The above and other objects are attained by the use of the present invention. The thermal rearrangement of salts of monocarboxylic acids having an aromatic structure into salts of diand polycarboxylic acids also produces very good yields at pressures of less than 400 atmospheres, provided the starting material is heated in the presence of salts of oxalic acid, advantageously in the presence of carbon dioxide or another inert gas and advantageously in the presence of catalytically active metals or metal compounds.

The aromatic monocarboxylic acids, Whose salts may serve as starting materials for the process according to the present invention are, for example, benzoic acid, 0tand ,B-naphthoic acid or diphenyl monocarboxylic acids.

Similarly, monocarboxylic acids in which the carboxyl groups are attached to another aromatic ring system, for example, to anthracene, terphenyl, diphenyl-methane or benzo-phenone, are suitable starting materials for the process according to the invention.

Furthermore, salts of heterocyclic monocarboxylic acids, the carboxyl groups of which are attached to heterocyclic rings having an aromatic structure may be used as starting materials. Such acids are derived, for example, from pyridine, quinoline, isoquinoline, a-pyrane, furan, thiophene, thiazole, indole, benztriazole or benzimldazole.

In all of these carboxylic acids, the aromatic ring or the heterocyclic ring having an aromatic structure may also carry other substituents in addition to the carboxyl groups, such as halogen atoms or alkyl radicals, provided the presence of such substituents does not cause a decomposition of the molecule below the reaction temperature.

The carboxylic acids mentioned above are employed in the form of their salts for the process according to the present invention. For this purpose, it is advantageous to employ the alkali metal salts, preferably the potassium salts as well as the sodium salts. The rubidium and cesium salts are also useful as such, but they must generally be excluded for economical reasons. The process may also be carried out with salts of other metals, for example, with alkaline earth metal salts of the abovementioned carboxylic acids. It is often advantageous to use mixtures of salts of two different metals, for example, mixtures of the sodium and potassium salts, because the mechanical properties of the reaction mixture are in many cases improved thereby. The reaction is influenced by the metal used in forming the carboxylic acid salt. For example, a mixture of potassium benzoate and potassium oxalate produces good yields of terephthalic acid and forms practically no side products. In contrast thereto, a mixture of sodium benzoate and sodium oxalate surprisingly forms orthophthalic acid as the principal product and trimesic acid and isophthalic acid as side products.

In place of the carboxylic acid salts themselves, reaction mixtures which yield such salts may be used. Mixtures of carboxylic acid anhydride or also of carboxylic acid esters and acid-binding metal compounds, such as alkali metal carbonates, are suited for this purpose. These mixtures need not be provided in stoichiometric ratio. One or the other components may be present in excess.

The oxalates used in the present process are preferably the oxalates of alkali metals, especially the oxalates of potassium or sodium. However, the oxalates of other metals, such as the alkaline earth metals, are also useful. The amount of oxalate added to the starting material may vary withinwide limits. Advantageously, however, this amount should not be less than one-half mol for each mol of aromatic carboxylic acid.

The salts or salt mixtures should preferably be used as starting materials in as dry a state as possible. if the salts are available in the form of aqueous solutions, they can be transformed into dry powders in accordance with wellknown procedures, preferably by spray-drying, and these dry powders may, if necessary, be subjected to further drying treatment to remove minute residual amounts of moisture.

We have further found that in order to achieve good yields, the presence of catalysts is required. Suitable catalysts are metals, especially cadmium, as well as zinc, mercury, lead and iron, and compounds of these metals, such as their oxides, their salts formed with inorganic or organic acids, their complex compounds and metal organic compounds; a few specific examples are their carbonates, bicarbonates, halides, sulfates, phosphates, acetates, formates, oxalates, fatty acid salts or also the salts of the above-mentioned metals formed with those acids which are used as starting materials for the reaction according to the invention or which are formed by this reaction, such as their benzoates or terephthalates. The amount of catalyst employed may vary within wide limits, for example, up to 15% by weight, preferably from 0.5 to by weight, based on the weight of the reaction mixture. The above-mentioned catalyst may also be employed in conjunction with known carriers such as Kieselguhr.

Especially good results are obtained if the catalyst is provided in a very finely divided form. This finely divided catalyst can be obtained, for example, by transforming an aqueous solution of the carboxylic acid salts serving as the starting material, which has the catalysts dissolved or suspended therein, into a dry powder by spray-drying or any other suitable manner. The catalyst may also be dissolved in molten potassium cyanate and the cooled and comminuted fused mass may be added to the starting material.

The reaction according to the present invention may not only be carried out in the presence of these catalysts but also in the presence of inert liquid or solid additives, for example, in the presence of sand, metal powders, metal shavings, Kieselguhr, activated charcoal, finely divided aluminum oxide, finely divided silicic acid or also in the presence of inert salts such as sodium sulfate. These additives in many cases improve the mechanical properties of the reaction mixture. In place of the solid inert materials, inert liquids which do not decompose under the prevailing conditions may be used as inert additives, such as toluene, benzene or the like.

It is further advantageous to carry out the reaction in the presence of carbon dioxide. Instead of carbon dioxide, gaseous mixtures may be used which contain other inert ases, such as nitrogen, methane or argon, in addition to carbon dioxide. The presence of oxygen should be avoided as much as possible. We have found that a carboxylation takes place even in the absence of carbon dioxide, for example, in an atmosphere of nitrogen. For example, the yields of terephthalic acid obtained in this manner from potassium benzoate are greater than that which would correspond to a disproportionation of the potassium benzoate into potassium terephthalate and benzene. The presence of carbon dioxide, however, produces a still further increase in the yield of terephthalic acid beyond that which would correspond to a disproportionation of, for example, potassium benzoate into potassium terephthalate and benzene.

The particular advantage of the process according to the present invention resides in that no high pressures are necessary. The reaction produces good yields at pressures below 100 atmospheres. Of course, the use of higher pressures is not detrimental.

As a rule, the reaction begins to proceed at temperatures above 300 C. The optimum reaction temperature, however, varies depending upon the starting materials used. For example, in the production of terephthalic acid from potassium benzoate and potassium oxalate the optimum reaction temperature lies in the neighborhood of 400 to 410 C. The upper temperature limit for this process is in general determined only by the decomposition temperature of the organic substances.

In order to avoid local overheating and decompositions caused thereby, as well as to avoid caking of the reaction mixture, it may be advantageous to maintain the reaction mixture in motion during the performance of the reaction according to this invention. This may be accomplished, for example, by heating the reaction mixture in vessels provided with a stirrer, in shaker autoclaves or in rotary autoclaves. Uniform distribution of the heat may, however, also be effected by distributing the reaction mixture in thin layers with or without agitation. However, good yields are also obtained without these particular provisions, provided care is taken that strong local overheating is avoided.

Th reaction mixture is worked up in accordance with known methods. The raw product is first dissolved in water or in dilute acids, and the solution is purified, if necessary, by filtration or by treatment with activated charcoal or other dis-coloring agents. Subsequently, the salts formed by the reaction can be transformed into the corresp nding free acids, for example, by acidifying the solution with organic or inorganic acids or by passing carbon dioxide at atmospheric pressure or elevated pressure therethrough. The oxalic acid present in the solution may be separated by crystallization or may be destroyed by oxidation. The free acids formed by the reaction can be separated from each other by maxing use of their different solubilities or volatiiities and may thus be isolated in pure form and thereafter may be transformed into their derivatives if desired. The carboxylic acid salt mixtures obtained from the reaction may also be transformed directly into derivatives of the acids, such as into their esters or halides, and these derivatives may be purified by fractional distillation.

The following specific examples are presented to illustrate our invention and to enable persons skilled in the art to better understand and practice the invention and are not intended to limit the invention.

Example 1 A mixture of 20.0 gm. potassium benzoate, 20.0 gm. anhydrous dipotassium oxalate and 5.0 gm. cadmium fluoride was intimately admixed by grinding in a ball mill and the resulting mixture was placed into a rotary autoclave having a volume of 200 cc. The autoclave was then flushed with carbon dioxide. Thereafter, carbon dioxide was introduced until the internal pressure reached 50 atmospheres and the autoclave was heated for one hour at 405 C. The maximum pressure which developed during the heating step was 180 atmospheres. After allowing the autoclave to cool and releasing the internal pressure, the raw reaction product was dissolved in 5 times its own volume of hot water, and the resulting solution was filtered to remove insoluble components. The clear filtrate was heated to the boiling point and was acidified at this temperature with hydrochloric acid. The terephthalic acid precipitated thereby was filtered off while the solution was still hot, and the filter cake was washed with hot water. The yield of terephthalic acid was 17.3 gm. In this example, for illustration if the reaction were merely that of disproportionation of potassium benzoate into terephthalic acid and benzene maximum yield of terephthalic acid to be expected would be 10.4 gm.

Example 2 A mixture of 10.0 gm. potassium benzoate, 30.0 gm. anhydrous dipotassium oxalate and 3.0 gm. cadmium fluoride was heated for 9 /2 hours at 400 C. in an atmosphere of carbon dioxide at an initial pressure of 50 atmospheres, in the same manner as described in Example 1. A maximum pressure of 150 atmospheres developed during the heating step. The raw product was worked up in the manner described in the previous example and yielded 7.6 gm. terephthalic acid.

Example 3 A mixture of 10.0 gm. sodium benzoate, 25.0 gm. sodium oxalate and 2.0 gm. cadmium fluoride was intimately admixed by grinding in a ball mill, and the resulting mixture was heated for 5 hours at 420 C. in a carbon dioxide atmosphere under a maximum pressure of atmospheres (at 420 C.), as described in Example 1. The raw reaction product which weighed 31.6 gm. was suspended in 500 cc. water and the resulting solution was filtered while hot. The content of sodium oxalate in the filtrate was analytically determined. It was 22.7 gm. The oxalate was destroyed by adding solid potassium permanganate at 70 C. to the solution which had previously been acidified with dilute sulfuric acid. The manganese dioxide formed thereby was removed with the aid of hydrogen peroxide. Subsequently, the solution was extracted with ether in a perforated tray column. By evaporation of the ether extract 5.8 gm. of a mixture of organic acids were obtained. This acid mixture was then digested with 75 cc. chloroform, whereby a portion of the acid mixture dissolved in the chloroform. The portion which was insoluble in chloroform weighed 1.5 gm. By infrared analysis it was determined that this insoluble portion consisted of orthophthalic acid and about 3% benzoic acid as an impurity. The portion which was soluble in chloroform (4.3 gm.) consisted of benzoic acid.

Example 4 A mixture of 10.0 gm. sodium benzoate, 25.0 gm. sodium oxalate and 2.0 gm. cadmium fluoride was intimately admixed by grinding in a ball mill and the resulting mixture was heated in a rotary autoclave having a net volume of 200 cc. for 3 hours at 362 C. in an atmosphere of carbon dioxide at an initial pressure of 50 atmospheres. A maximum pressure of 132 atmospheres developed during the heating step. The raw reaction product which weighed 34.8 gm. was dissolved in one liter hot water and the solution was filtered to remove the undissolved catalyst. The filtrate was acidified with dilute sulfuric acid. Upon cooling a precipitate formed which was separated by suction filtration and isolated. The precipitate weighed 5.0 gm. and consisted of 3.15 gm. acid sodium oxalate and 1.85 gm. benzoic acid. By extraction of the filtrate with ether, 4.0 gm. of a mixture of organic acids were obtained. Upon digestion of this mixture with chloroform, 1.8 gm. benzoic acid were separated. The remainder of 2.2 gm. consisted of orthophthalic acid.

Example 5 A mixture of 20.0 gm. potassium benzoate, 30.0 gr potassium oxalate and 10.0 gm. of a melt, produced from 10.0 gm. potassium cyanate'and 3.0 gm. cadmium fluoride, was intimately admixed by milling and the resulting mixture was heated in a rotary autoclave having a net volume of 200 cc. for one hour at 415 C. in an atmosphere of nitrogen under a pressure of 180 atmospheres. The raw reaction product was worked up in the manner described in Example 1. 10.7 gm. terephthalic acid were obtained.

Example 6 A mixture of 20.0 gm. potassium benzoate, 30.0 gm. potassium oxalate and 10.0 gm. of a melt, produced from 10.0 gm. potassium cyanate and 3.0 gm. of cadmium carbonate was intimately admixed by milling in a ball mill and theresulting mixture was heated in a rotary autoclave having a net volume of 200 cc. for 2 hours at 405 C. in an atmosphere of carbon dioxide at a pressure of 150 atmospheres. The reaction mixture was worked up in the manner described above. 17.7 gm. terephthalic acid were obtained.

The same starting material was subjected to the heat treatment at various reaction temperatures and varying reaction periods. The following yields were obtained:

A mixture of 20.0 gm. potassium benzoate, 30.0 gm. potassium oxalate and 10.0 gm. of a melt produced from 10.0 gm. potassium cyanate and 3.0 gm. zinc carbonate was intimately admixed by milling and the resulting mixture was heated in a rotary autoclave having a net vol 6 ume of 200 cc. for one hour at 430 C. in an atmosphere of purified nitrogen at a pressure of 152 atmospheres. The raw reaction product was worked up in the manner described in Example 1. 11.7 gm. terephthalic acid were obtained.

Example 8 A mixture of 20.0 gm. potassium benzoate, 10.0 gm. anhydrous dipotassium oxalate, 10.0 gm. calcium oxalate and 3.0 gm. cadmium fluoride was intimately admixed by milling in a ball mill and the resulting mixture was placed into a rotary autoclave having a net volume of 200 cc. The autoclave was flushed with carbon dioxide. Thereafter, carbon dioxide was introduced into the autoclave under pressure until the internal pressure reached 50 atmospheres, and the autoclave was then heated for one hour at 405 C. After cooling and releasing the pressure from the autoclave, the reaction product was worked up in the same manner described in Example 1. 12.1 gm. terephthalic acid were obtained.

Example 9 A mixture of 20.0 gm. potassium benzoate, 20.0 gm. dipotassium oxalate and 10.0 gm. anhydrous zinc oxalate was heated for one hour at 405 C. under the same conditions as described in Example 8. Upon working up the reaction mixture in the manner described in Example 1, 10.8 gm. terephthalic acid were obtained.

Example 10 A mixture of 20.0 gm. of the potassium salt of nicotinic acid, 30.0 gm. potassium oxalate and 3.0 gm. anhydrous cadmium fluoride was heated for 10 hours at 350 C. in an autoclave having a net volume of 0.2 liter in an atmosphere of carbon dioxide at an initial pressure of 50 atmospheres. After cooling and releasing the pressure rrom the autoclave the reaction mixture was dissolved in 250 cc. water. The solution was filtered and the filtrate was purified by treatment with activated charcoal in the customary manner. Subsequently, the solution was acidi led with hydrochloric acid. After allowing the acidified solution to stand in the cold state, the precipitated crystals were separated by suction filtration. The mother liquor was extracted with ether. All together, 11.4 gm. pyridine-2,5-dicarboxylic acid were obtained.

Example 11 A mixture of 20.0 gm. rubidium benzoate, 20.0 gm. anhydrous dipotassiurn oxalate and 5.0 gm. cadmium fluoride was intimately admixed by milling in a ball mill and the resulting mixture was placed in a rotary autoclave having a net volume of 200 cc. The autoclave was flushed with carbon dioxide. Thereafter, carbon dioxide was introduced into the autoclave under pressure until the internal pressure reached 50 atmospheres, whereupon the autoclave and its contents were heated at 400 C. for one hour. The raw reaction product was dissolved in water and the solution was then worked up as described in Example 1. 13.1 gm. terephthalic acid were obtained.

By the use of the same method and conditions set forth in the above examples, carboxyl groups may be introduced into the ring structure of other aromatic compounds, such as naphthoic acid, diphenyl monocarboxylic acids, or monocarboxylic acids derived from diphenyl, anthracene, terphenyl, diphenyl-methane, benzophenone, pyridine, quinoline, isoquinoline, ot-pyrane, furan, thiophene, thiazole, indole, benztriazole or benzimidazole.

Example 12 A mixture of 50.0 gm. sodium benzoate, 5.0 gm. sodium oxalate and 2.5 gm. cadmium fluoride was heated for 5 hours at 440 C. in a rotary autoclave having a volume of 200 cc. in an atmosphere of carbon dioxide under a pres sure of 155 atmospheres at reaction temperature. After cooling and releasing the autoclave the raw reaction product was dissolved in 400 cc. of water. The solution was then filtered and acidified with sulfuric acid. A precipitate was formed which however did not injure the further working up. The oxalate which had not reacted was now destroyed by adding solid potassium permanganate. The surplus of potassium permanganate and the manganese were reduced and dissolved with hydrogen peroxide. Thereafter the solution was cooled with ice together with the precipitate obtained on acidifying. 4.9 gm. of a mixture of the monopotassium salt of trimesic acid and of free trimesic acid precipitated out and were separated ofi. The mother liquor was extracted With ether in a perforated tray column. The ether solution was evaporated and the residue was digested with chloroform. 7.2 gm. orthophthalic acid and 8.6 gm. benzoic acid were obtained in the same manner as described in Example 3.

In order to produce alkali metal salts of naphthalene dicarboxylic acids using the same method as described in the previous examples, alkali metal salts of aor [3- naphthoic acid were heated to a temperature of above 300 C., but below the decomposition temperature of the starting materials or products, in a substantially oxygen free inert atmosphere, in the presence of alkali or alkaline earth metal salts of oxalic acid and a cadmium containing catalyst under a pressure of less than 400 atmospheres for a time to eiTect the conversion of the monocarboxylic acid salt to the dicarboxylic salt. The yield of dicarboxylic product was greater than 50% of the monocarboxylate starting material.

While We have set forth a number of specific embodiments and preferred modes of practice of our invention, it will be understood that the invention is not limited thereto, and that various modifications may be made without departing from the spirit of the disclosure or the scope of the appended claims.

We claim:

1. The method of producing alkali metal salts of pyridine-2,5-dicarboxylic acid by the thermal conversion of nicotinic acid, which comprises heating an alkali metal salt of nicotinic acid to a temperature above 300 C. and below the temperature at which said salt and the reaction products substantially decompose, in a substantially oxygen free inert atmosphere, in the presence of salts of oxalic acid, said salts being selected from the group consisting of alkali and alkaline earth metal salts, in the presence of a catalyst containing a material from the group consisting of cadmium, zinc, mercury, lead and iron and the oxides and salts of said metals, and under a pressure of less than 400 atmospheres for a time sufficient to effect said conversion, thereby obtaining a molar yield of said pyridine-2,S-dicarboxylic acid which is more than of the nicotinic acid salt undergoing conversion.

2. In the method of producing alkali metal salts of naphthalene dicarboxylic acids by the thermal conversion of the corresponding naphthalene monocarboxylic acid salts, which comprises heating the naphthalene monocarboxylic salts to be converted to a temperature above 300 C. and below the temperature at which said salt and the reaction products substantially decompose in a substantially oxygen free inert atmosphere the improvement which comprises conducting the reaction, in the presence of salts of oxalic acid, said salts being selected from the group consisting of alkali and alkaline earth metal salts, in the presence of a cadmium containing catalyst, and under a pressure of less than 400 atmospheres for a time sufficient to elfect said conversion, thereby obtaining a molar yield of the naphthalene dicarboxylic acid which is more than 50% of the naphthalene monocarboxylic acid salts undergoing conversion.

References Cited in the file of this patent UNITED STATES PATENTS 2,794,830 Raecke et al June 4, 1957 2,823,229 Raecke et al Feb. 11, 1958 2,823,230 Raecke et al Feb. 11, 1958 3,043,846 Blaser et al July 10, 1962 3,101,368 Schenk Aug. 20, 1963 

1. THE METHOD OF PRODUCING ALKALI METAL SALTS OF PYRIDINE-2,5-DICARBOXYLIC ACID BY THE THERMAL CONVERSION OF NICOTINIC ACID, WHICH COMPRISES HEATING AN ALKALI METAL SALT OF NICOTINIC ACID TO A TEMPERTURE ABOVE 300*C. AND BELOW THE TEMPERATURE AT WHICH SAID SALT AND THE REACTION PRODUCTS SUBSTANTIALLY DECOMPOSE, IN A SUBSTANTIALLY OXYGEN FREE INERT ATMOSPHERE, IN THE PRESENCE OF SALTS OF OXALIC ACID, SAID SALTS BEING SELECTED FROM THE GROUP CONSISTING OF ALKALI AND ALKALINE EARTH METAL SALTS, IN THE PRESENCE OF A CATALYST CONTAINING A MATERIAL FROM THE GROUP CONSISTING OF CADMIUM, ZINC, MERCURY, LEAD AND IRON AND THE OXIDES AND SALTS OF SAID METALS, AND UNDER A PRESSURE OF LESS THAN 400 ATMOSPHERES FRO A TIME SUFFICIENT TO EFFECT SAID CONVERSION, THEREBY OBTAINING A MOLAR YIELD OF SAID PYRIDINE-2,5-DICARBOXYLIC ACID WHICH IS MORE THAN 50% OF THE NICOTINIC ACID SALT UNERGOING CONVERSION. 